While not limited thereto, the present invention is particularly adapted for use in pneumatic rock drills of the type in which a rotating drill rod is reciprocated by means of a hammer piston which repeatedly strikes the end of the drill rod opposite a drill bit. In such drills, the hammer piston delivers a blow upon the drill rod at one end of its downward stroke, the initial propulsion of the piston toward the rod being due to expansion of a fluid under pressure; while the final portion of the stroke is due to the inertia of the piston and any residual pressure in the cylinder after the fluid pressure source has been cut off. Similarly, the upward stroke of the hammer piston is initiated by fluid under pressure; while the remainder of the upward stroke is due to the inertia of the piston and/or any residual pressure in the cylinder.
In the past, many pneumatic motors of this type have required a plurality of input ports extending through the wall of a cylinder which houses the hammer piston. This, however, increases the cost of the assembly. Furthermore, prior art designs as exemplified, for example, in U.S. Pat. Nos. 873,938, 1,128,416, 1,660,201, 1,800,344, 2,748,750 and 3,329,068 have been deficient in one respect or another as regards the efficiency and cost of the pneumatic motor.